A firewall set up late Tuesday by Facebook (Nasdaq: FB), the No. 1 social networking site, appears to be holding: shares rose 45 cents to $18.17 in early Wednesday after CEO Mark Zuckerberg promised not to sell any more shares for at least a year.

On Tuesday, shares of the Menlo Park, Calif.-based website set a post-initial public offering low of $17.55, less than half the value of their $38 price on May 17.

Two more insiders, Marc Andreesen, a venture capitalist and director of Hewlett-Packard Co. (NYSE: HPQ), and Donald Graham, chairman of the Washington Post Co. (NYSE: WPO), also promised not to sell any more shares. In the past two weeks, insiders Peter Thiel and Dustin Moskovitz have sold more than 21 million shares, helping to send the price down.

The firewall also includes a pledge by Facebook to buy back 101 million shares from early employees, a deal that could cost as much as $1.9 billion. The company raised $16 billion in its IPO and reported holding about $10 billion on June 30.

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